Ptfe layers and methods of manufacturing

ABSTRACT

Thin PTFE layers are described having little or no node and fibril microstructure and methods of manufacturing PTFE layers are disclosed that allow for controllable permeability and porosity of the layers. In some embodiments, the PTFE layers may act as a barrier layer in an endovascular graft or other medical device.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/251,011, filed Aug. 30, 2016, which is a continuation of U.S. patentapplication Ser. No. 14/230,318, filed Mar. 31, 2014, now U.S. Pat. No.9,446,553, which is a continuation of U.S. patent application Ser. No.12/915,636, filed Oct. 29, 2010, now U.S. Pat. No. 8,728,372, which is acontinuation of U.S. patent application Ser. No. 12/250,946, filed Oct.14, 2008, abandoned, which is a continuation of U.S. patent applicationSer. No. 11/106,150, filed Apr. 13, 2005, abandoned, the contents ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Polytetrafluoroethylene (PTFE) layers have been used for the manufactureof various types of intracorporeal devices, such as vascular grafts.Such vascular grafts may be used to replace, reinforce, or bypass adiseased or injured body lumen. One conventional method of manufacturing“expanded” PTFE layers is described in U.S. Pat. No. 3,953,566 by Gore.In the methods described therein, a PTFE paste is formed by combining aPTFE resin and a lubricant. The PTFE paste may be extruded. After thelubricant is removed from the extruded paste, the PTFE article isstretched to create a porous, high strength PTFE article. The expandedPTFE layer is characterized by a porous, open microstructure that hasnodes interconnected by fibrils.

Such an expansion process increases the volume of the PTFE layer byincreasing the porosity, decreasing the density and increasing theinternodal distance between adjacent nodes in the microstructure whilenot significantly affecting the thickness of the PTFE layer. As such,the conventional methods expand the PTFE layer and impart a porosity andpermeability while only providing a negligible reduction in a thicknessof the PTFE layer. In situations where a thin PTFE layer, andspecifically, a thin PTFE layer having a low fluid permeability isneeded, conventional PTFE layers are largely unsatisfactory due to theporosity and highly permeable nature of the expanded PTFE layer.

Therefore, what has been needed is improved PTFE layers and improvedmethods for manufacturing the PTFE layers. In particular, it would bedesirable to have thin PTFE layers that have a controllable permeabilityto fluids (gases, liquids or both). It may also be desirable to havesuch thin PTFE layers that have a high degree of limpness and supplenessto allow mechanical manipulation or strain of such a PTFE layer withoutsignificant recoil or spring back.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention provide PTFE layers and films andmethods of manufacturing the PTFE layers and films. Embodiments of thepresent invention may include one or more layers of a fluoropolymer,such as PTFE. Embodiments of PTFE layers may include at least a portionthat does not have a significant or discernable node and fibrilmicrostructure.

In one embodiment, a method of processing PTFE includes providing alayer of PTFE, applying stretching agent to at least a portion of thelayer of PTFE and stretching the layer of PTFE while the layer of PTFEis wet with stretching agent. In another embodiment, a method ofprocessing PTFE includes providing a layer of PTFE, applying stretchingagent to at least a portion of the layer until a saturated portion ofthe surface is saturated with stretching agent and stretching the layerof PTFE while the layer of PTFE is saturated with stretching agent. Inanother embodiment, a method of processing PTFE includes providing astretched layer of PTFE that has been stretched in at least a firstdirection, applying stretching agent to at least a portion of thestretched layer and stretching the stretched layer of PTFE while thelayer of PTFE is wet with stretching agent. Also, for some embodiments,the direction of the first direction and the direction of the secondstretch may be substantially the same or different. For example, in oneembodiment, the first direction is the machine direction and the secondstretch is carried out or performed in the transverse direction. Inanother embodiment, the first direction is the machine direction and thesecond stretch is carried out in substantially the same machinedirection. In other embodiments, the first direction may be a transversedirection. Also, for some embodiments, the stretch in the firstdirection may have been carried out with sufficiently low stretchingagent content so as to produce a significant or discernable node andfibril microstructure during the stretch in the first direction. Inother embodiments, the stretch in the first direction may have beencarried out while the layer of PTFE was wet with stretching agent to theextent that little or no node and fibril microstructure was createdduring the stretch in the first direction.

In another embodiment, a method of processing PTFE includes providing alayer of PTFE, applying stretching agent to at least a portion of thelayer of PTFE, stretching the layer of PTFE while the layer of PTFE iswet with stretching agent, stretching the stretched layer of PTFE asecond time and calendering the twice stretched layer of PTFE so as todensify, compress and further thin the material. Another embodiment isdirected to a method of processing PTFE including providing a layer ofPTFE, applying stretching agent to at least a portion of the layer untilat least a portion of the layer is saturated with the stretching agentto form a saturated portion and stretching the layer of PTFE. Otherembodiments include PTFE layers made by any combination of the methodsdiscussed above.

Regarding layer embodiments, one layer embodiment is directed to a thinPTFE layer having low porosity, low fluid permeability, substantially nonode and fibril structure, and having a thickness of about 0.00005 inchto about 0.005 inch. Another embodiment is directed to a thin PTFElayer, having substantially low porosity, substantially low fluidpermeability, substantially no node and fibril structure, and a highdegree of limpness and suppleness so to allow mechanical manipulation orstrain of the PTFE layer without significant recoil or spring back.

In another embodiment, a PTFE composite film comprises a first layerincluding a stretched layer of PTFE that has a closed cellmicrostructure with a plurality of interconnected high density regionssubstantially free of node and fibril microstructure between the highdensity regions. The PTFE composite film also comprises a second layerof expanded PTFE which is secured to the first layer and which includesnode and fibril microstructure. In another embodiment, a thin fluid-PTFElayer having low or substantially no fluid permeability is produced byproviding a PTFE layer, adding a stretching agent to the PTFE layer andstretching the PTFE layer in at least one direction to reduce athickness of the PTFE layer. In another embodiment, a thin layer of PTFEincludes a stretched layer of PTFE that has a closed cell microstructurewith a plurality of interconnected high density regions substantiallyfree of node and fibril microstructure between the high density regions.

Another embodiment is directed to a multi-layered vascular graft thatincludes a first tubular body having an outer surface and an innersurface that defines an inner lumen of the vascular graft and a secondtubular body having an outer surface and an inner surface coupled to theouter surface of the first tubular body. In this embodiment, one of thefirst tubular body and the second tubular body includes afluid-permeable PTFE layer, and the other tubular body comprises afluid-PTFE layer having low or substantially no fluid permeability. Inanother embodiment, an inflatable endovascular graft includes a bodyportion having an inflatable channel that defines an inflatable space.The inflatable space of this embodiment is at least partially surroundedby a thin PTFE layer having low or substantially no fluid permeability.

Another embodiment is directed to a stretched PTFE layer having low orsubstantially no fluid permeability that includes a closed cellmicrostructure having high density regions whose grain boundaries aredirectly interconnected to grain boundaries of adjacent high densityregions and having substantially no node and fibril microstructure. Inanother embodiment, a composite film includes a fluid-permeable,expanded PTFE layer secured to a surface of a thin stretched PTFE layerhaving a closed cell microstructure, having high density regions whosegrain boundaries are directly interconnected to grain boundaries ofadjacent high density regions and having substantially no node andfibril microstructure.

Another embodiment is directed to a tubular structure having a compositefilm with a fluid-permeable, expanded PTFE layer secured to a surface ofa thin, stretched PTFE layer. The thin, stretched PTFE layer has aclosed cell microstructure with high density regions whose grainboundaries are directly interconnected to grain boundaries of adjacenthigh density regions and with substantially no node and fibrilmicrostructure. In another embodiment, an endovascular graft includes acomposite film with a fluid permeable, expanded PTFE layer secured to asurface of a thin stretched PTFE layer. The stretched PTFE layer has aclosed cell microstructure with high density regions whose grainboundaries are directly interconnected to grain boundaries of adjacenthigh density regions and with substantially no node and fibrilmicrostructure.

In another embodiment, a thin PTFE layer has substantially low porosity,low fluid permeability, substantially no node and fibril structure, anda high degree of limpness and suppleness so to allow mechanicalmanipulation or strain of the PTFE layer without significant recoil orspring back. In another embodiment, a thin layer of PTFE includes astretched layer of PTFE that has a closed cell microstructure with aplurality of interconnected high density regions substantially free ofnode and fibril microstructure between the high density regions. Inanother embodiment, a method of controlling the porosity, density orboth of a PTFE layer, includes stretching the PTFE layer at least onetime at a preselected temperature and preselected stretching agentcontent for the at least one stretch.

These features of embodiments will become more apparent from thefollowing detailed description when taken in conjunction with theaccompanying exemplary drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a ram extruder extruding a PTFE ribbon that is beingtaken up on a spool.

FIG. 2 illustrates a calendering process of the PTFE ribbon of FIG. 1.

FIGS. 3 and 4 illustrate a tentering process with stretching agent beingapplied to a PTFE layer during the stretching process.

FIGS. 5 and 6 illustrate a machine direction stretching process of thestretched PTFE layer of FIGS. 3 and 4.

FIGS. 7 and 8 illustrate a final calendering or densification processperformed on a stretched PTFE layer.

FIG. 9 is a scanning electron microscope (SEM) image of a PTFE layer ata magnification of 20,000.

FIG. 10 is a SEM image of the PTFE layer of FIG. 9 at a magnification of14,000.

FIG. 11 is a SEM image of the PTFE layer of FIG. 9 at a magnification of7,000.

FIG. 12 is a SEM image of the PTFE layer of FIG. 9 at a magnification of3,000.

FIG. 13 is a SEM image of the PTFE layer of FIG. 9 at a magnification of500.

FIG. 14 schematically illustrates a composite PTFE film that comprises aPTFE layer having low or substantially no fluid permeability and aporous PTFE layer.

FIG. 15 schematically illustrates a simplified tubular structure thatcomprises an outer layer having low or substantially no fluidpermeability and a fluid-permeable inner layer.

FIG. 16 schematically illustrates a simplified tubular structure thatcomprises a layer having low or substantially no fluid permeability anda fluid-permeable outer layer.

FIG. 17 illustrates an embodiment of an endovascular graft having anetwork of inflatable conduits.

FIGS. 18 to 20 are transverse cross sectional views of an inflatableconduit of the graft of FIG. 17.

FIG. 21 is a transverse cross sectional view of an embodiment of atubular inflatable conduit.

FIG. 22 is an elevational view that illustrates another embodiment of aninflatable endovascular graft.

FIG. 23 illustrates an embodiment of an inflatable bifurcatedendovascular graft.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention relate generally to thin PTFElayers, PTFE films, composite films having two or more PTFE layers andmethods of manufacturing the PTFE layers, films and composite films.Some particular embodiments are directed to thin PTFE layers having lowor substantially no fluid permeability with a microstructure that doesnot include significant fibril and nodal structure as is common withexpanded PTFE layers. It may also be desirable for some embodiments ofsuch thin PTFE layers that have a high degree of limpness and supplenessso to allow mechanical manipulation or strain of such a PTFE layerwithout significant recoil or spring back. Such PTFE layers may bemanufactured and used for construction of endovascular grafts or othermedical devices. For some applications, embodiments of PTFE films mayinclude one or more discrete layers of PTFE that are secured together toform a composite film. As used herein, the term “composite film”generally refers to a sheet of two or more PTFE layers that havesurfaces in contact with each other, and in some embodiments, may besecured to each other such that the PTFE layers are not easilyseparated. The individual PTFE layers used in some of the PTFE compositefilm embodiments herein may have the thinness and low fluid permeabilitycharacteristics discussed above in combination with other layers havingthe same or different properties. Some PTFE layer embodiments have a lowfluid permeability while other PTFE layer embodiments have no orsubstantially no fluid permeability. A PTFE layer having a low fluidpermeability may, for some embodiments, be distinguished from thepermeability of a standard layer of expanded PTFE by comparing fluidpermeability based on Gurley test results in the form of a Gurley Numberor “Gurley Seconds”. The Gurley Seconds is determined by measuring thetime necessary for a given volume of air, typically, 25 cc, 100 cc or300 cc, to flow through a standard 1 square inch of material or filmunder a standard pressure, such as 12.4 cm column of water. Such testingmaybe carried out with a Gurley Densometer, made by Gurley PrecisionInstruments, Troy, N.Y. A standard porous fluid permeable layer ofexpanded PTFE may have a Gurley Number of less than about 15 seconds,specifically, less than about 10 seconds, where the volume of air usedis about 100 cc. In contrast, embodiments of layers of PTFE discussedherein having low fluid permeability may have a Gurley Number of greaterthan about 1500 seconds where 100 cc of air is used in the test. Anembodiment of a PTFE layer discussed herein having no or substantiallyno fluid permeability may have a Gurley Number of greater than about 12hours, or up to a Gurley Number that is essentially infinite, or toohigh to measure, indicating no measurable fluid permeability. Some PTFElayer embodiments having substantially no fluid permeability may have aGurley Number at 100 cc of air of greater than about 1×10⁶ seconds.Stretched PTFE layers processed by embodiments of methods discussedherein having no discernable node or fibril microstructure may initiallyhave substantially no fluid permeability. However, such PTFE layerembodiments may subsequently be stretched during a manufacturingprocess, such as the manufacture of an inflatable endovascular graft,during which process the PTFE layer may become more fluid permeable andachieve a level of low permeability as discussed above.

FIGS. 1-8 illustrate processing of PTFE material to form a thin,stretched PTFE layer having low or substantially no fluid permeabilityfor particular fluids. As such, embodiments of the stretched PTFE layersare not “expanded” in the conventional sense as taught by Gore in, e.g.,U.S. Pat. No. 3,953,566. For example, the stretched PTFE layers may besubstantially thinned during stretching whereas prior art “expansion”processes typically leave the thickness of the expanded materialsomewhat unchanged but generate distinct nodal and fibril microstructurealong with increased porosity and permeability in order to accommodatethe expansion of the layer in plane of the layer.

Referring to FIG. 1, a fine PTFE resin powder is compounded with anextrusion agent such as a liquid lubricant to form a PTFE compound 10. Avariety of different PTFE resins may be used such as the lower extrusionratio, higher molecular weight fine powder coagulated dispersion resins(available from 3M Corporation, Ausimont Corporation, DaikinCorporation, DuPont and ICI Corporation). The PTFE molecules used inthese resins typically have an average molecular weight of from about 20million to about 50 million or more. Optionally, an additive, such aspowdered or liquid color pigment or other resin additive may be added tothe PTFE resin and lubricant to change the properties of the final PTFElayer. For example, a fluorinated copolymer may be added (such asperfluoropropylvinylether-modified PTFE) to improve the bondability ofthe PTFE layer. Additive is typically provided in a mass amount that isless than 2% of the mass of the PTFE resin, but it may be provided inany amount that produces a desired result. Additive may be combined withthe PTFE resin before the lubricant is added so as to ensure homogenousmixing of the additive throughout the PTFE resin.

A variety of different types of extrusion and stretching agents, orlubricants, may be compounded with the PTFE powder resin. Some examplesof lubricants that may be mixed with the PTFE resin include, but are notlimited to, isoparaffin lubricants such as ISOPAR® H, ISOPAR® K andISOPAR® M all of which are manufactured by ExxonMobil Corporation.Additional lubricants include mineral spirits, naphtha, MEK, toluene,alcohols such as isopropyl alcohol, and any other chemical that iscapable of saturating the PTFE resin. In addition, two or morelubricants may be blended together for some lubricant embodiments. Theamount of lubricant added to the PTFE resin may vary depending on thetype of lubricant used as well as the desired properties of a final PTFElayer. Typically, however, the percent mass of lubricant for somecompound embodiments may vary from about 15% to about 25% of thecompound mass, specifically, from about 17% to about 22% of the compoundmass, and more specifically from about 18% to about 20% of the compoundmass.

The PTFE resin and lubricant may be mixed until a substantiallyhomogenous PTFE compound 10 is formed. Compounding of the PTFE resin andlubricant is typically carried out at a temperature below the glasstransition temperature of the PTFE resin which is typically from about55° F. to about 76° F. Compounding of the PTFE resin may be carried outat a temperature below about 50° F., and specifically, at a temperatureof from about 40° F. to about 50° F., so as to reduce shearing of thefine PTFE particles. Once mixed, the PTFE compound may be stored at atemperature of above approximately 100° F., and typically from about110° F. to about 120° F. for a time period that ensures that thelubricant has absorbed through the PTFE resin particles. The storagetime period typically may be greater than about six hours, and may varydepending on the resin and lubricant used.

Once the compounded PTFE resin and lubricant 10 have been suitablyprepared, the compound 10 may be placed in an extruder, such as the ramextruder 12 shown in FIG. 1. The ram extruder 12 includes a barrel 13and a piston 14 that is configured to slide within a chamber of thebarrel 13 and form a seal against an inner cylindrical surface of thebarrel 13. The compound 10 is placed in the chamber of the extruder 12between the distal end of the piston 14 and an extruder die 16 sealed tothe output end 18 of the extruder 12. The ram extruder 12 may alsoinclude heat elements 20 disposed about the output end 18 of the barrel13 which are configured to uniformly heat the output end 18 of theextruder 12. In some methods, the output end 18 of the extruder isheated before the compounded PTFE resin 10 is loaded into the chamber.An embodiment of a ram extruder 12 may include a Phillips ScientificCorporation vertical three inch hydraulic ram extruder.

Once the PTFE resin compound is loaded, the piston 14 is advancedtowards the output end 18 of the extruder 12, as indicated by arrow 21,which increases the chamber pressure and forces the PTFE compound 10 tobe extruded through an orifice 22 of the die 16 to form an extrudate 24.The extrudate 24 may be in the form of a ribbon or tape that is thenwound onto a take up spool 26 as indicated by the arrow adjacent thetake up spool in FIG. 1. The ram extrusion process represents amechanical working of the compound 10 and introduces shear forces andpressure on the compound 10. This working of the compound results in amore cohesive material in the form of extrudate ribbon or tape 24.

Processing conditions may be chosen to minimize the amount of lubricantthat is evaporated from the PTFE extrudate ribbon 24. For example, thePTFE compound 10 may be extruded at a temperature that is above theglass transition temperature, and typically above about 90° F. The PTFEextrudate ribbon 24 is generally fully densified, non-porous andtypically has approximately 100% of its original amount of lubricantremaining upon extrusion from the die 16. The die 16 may also beconfigured to produce an extrudate 24 having other configurations, suchas a tubular configuration. Also, for some methods, the PTFE compound 10may be processed to form a preform billet before it is placed in theextruder 12. In addition, a de-ionizing air curtain optionally may beused to reduce static electricity in the area of the extruder 12. In oneexample, the ram extruder 12 has a barrel 13 with a chamber having aninside transverse diameter of about 1 inch to about 6 inches indiameter. Embodiments of the die 16 may have orifices 22 configured toproduce an extrudate ribbon or tape 24 having a width of about 1 inch toabout 24 inches and a thickness of about 0.020 inch to about 0.040 inch,specifically, about 0.025 inch to about 0.035 inch.

After extrusion, the wet PTFE extrudate ribbon 24 may be calendered in afirst direction or machine direction, as indicated by arrow 27, toreduce the thickness of the PTFE extrudate ribbon 24 into a PTFE layer28 as shown in FIG. 2. During the calendering process, the width of thePTFE extrudate ribbon 24 and calendered PTFE layer 28 changes littlewhile the PTFE extrudate ribbon 24 is lengthened in the machinedirection. In one embodiment, the PTFE extrudate ribbon 24 andcalendered PTFE layer 28 may be about 6 inches to about 10 inches inwidth. The calendering process both lengthens and reduces the thicknessof the PTFE ribbon 24 to form PTFE layer 28 that is taken up by spool32. During calendering, the PTFE extrudate ribbon 24 may be calenderedbetween adjustable heated rollers 30 to mechanically compress and reducethe thickness of the PTFE ribbon 24. As such, the calendering processalso encompasses a second mechanical working of the compound 10.Suitable equipment for the calendering process includes a custom 12 inchvertical calender machine manufactured by IMC Corporation, Birmingham,Ala.

While it may be possible to store the PTFE extrudate ribbon 24 for anextended period of time after extrusion, lubricant in the PTFE extrudateribbon 24 will evaporate from the ribbon 24 during the storage period.As such, it may be desirable in some instances to calender the PTFEextrudate ribbon 24 almost immediately after extrusion so as to bettercontrol the lubricant level in the PTFE extrudate ribbon 24. For someembodiments, the PTFE ribbon 24 will have a lubricant content of about15% to about 25% immediately prior to calendering.

Depending on the calendering speed and roller positioning, the PTFEribbon 24 may be calendered down to produce a PTFE layer 28 of anysuitable thickness. The reduction ratio of an embodiment of thecalendering process, which is a ratio of the thickness of the PTFEextrudate ribbon 24 to the thickness of the calendered PTFE layer 28,maybe from about 3:1 to about 75:1, and specifically from about 7.5:1 toabout 15:1. In one particular embodiment, for a PTFE extrudate ribbon 24having a thickness of about 0.030 inch, calendering may reduce itsthickness to about 0.001 inch to about 0.006 inch, specifically, fromabout 0.002 inch to about 0.004 inch. In some instances, the PTFE ribbon24 may be calendered to a PTFE layer 28 which has a thickness that isslightly greater than a final desired thickness, so that the finalstretch of the PTFE ribbon 24 causes the final PTFE layer 28 to have itsdesired thickness.

The calendering temperatures and processing parameters may be chosen sothat the calendered PTFE layer 28 still has a significant amount ofresidual lubricant after the calendering process. For this embodiment,the adjustable rollers 30 may be heated to a temperature from about 100°F. to about 200° F., and specifically from about 120° F. to about 160°F. during the calendering process. After calendering, a residual amountof lubricant will remain in the PTFE layer 28 which typically may befrom about 10% to about 22% lubricant by weight remaining, specificallyabout 15% to about 20% lubricant by weight.

Once the PTFE ribbon 24 has been calendered to produce PTFE layer 28,PTFE layer 28 then may be mechanically stretched transversely (alsocalled the cross machine direction), in the longitudinal direction (alsocalled the machine direction), in both of these directions or any othersuitable direction or combination of directions, in order to thin thePTFE layer 28, generate a suitable microstructure and mechanically workthe PTFE. It should be noted that although this specification describesa process whereby a PTFE layer is stretched transversely, then stretchedlongitudinally and then densified, the order these steps are performedin may be changed. For example, a PTFE layer may be first stretchedlongitudinally then stretched transversely. Such a layer optionally maythen be densified as discussed below. For the transverse stretchingprocess shown in FIGS. 3 and 4, a tentering machine 34 may be used tomechanically stretch the calendered PTFE layer 28 into a stretched PTFElayer 36. One embodiment of a suitable tentering machine 34 includes a60 inch wide by 28 foot long tenter having a T-6 10 horsepower driveunit, manufactured by Gessner Industries, Concord, N.C.

For some embodiments, in order to produce the desired combination of anyof thickness, porosity, fluid permeability as well as mechanicalproperties, process parameters such as temperature, stretch ratios andmaterial lubricant content of PTFE layer 28 may be controlled before andduring the stretching process of the PTFE layer. As such, for someembodiments, a stretching agent or lubricant 40 optionally may beapplied to the calendered PTFE layer 28 during the stretching process asshown in FIGS. 3 and 4. Applying the stretching agent 40 to the PTFElayer 28 prior to or during the stretching process of the PTFE layer 28may be used to control the lubricant content of the stretched PTFE layer36. This technique may be used to impart particular characteristics tothe stretched PTFE layer 36 such as thinness, low porosity and low orsubstantially no fluid permeability. This method embodiment also allowsfor the stretched PTFE layer 36 to have a high degree of limpness andsuppleness so to allow mechanical manipulation or strain of such a PTFElayer without significant recoil or spring back which may beparticularly useful for some applications. If a high density,liquid-impermeable and gas-impermeable PTFE layer 28 having low orsubstantially no fluid permeability is desired, the PTFE layer 28 may besaturated throughout the thickness of the PTFE layer 28 with one or morestretching agents 40 during stretching. If a more porous PTFE layer 28is desired, a lesser amount of stretching agent 40 will be applied ontothe PTFE layer 28. Stretching the PTFE layer 28 may be carried out forsome embodiments at a temperature of about 80° F. to about 100° F.,specifically, about 85° F. to about 95° F.

The stretching agent 40 may be the same lubricant used to form the PTFEcompound 10 or it may be a different lubricant or combination oflubricants. In some embodiments, the stretching agent may be applied insufficient quantities to the PTFE layer 28 to saturate the PTFE layer 28during the stretching process. The stretching agent maybe applied by avariety of methods to a surface, such as the upper surface 38, of thePTFE layer 28 during the stretching process. For example, the stretchingagent 40 may be sprayed over the entire layer 28 or only on selectedportions of the PTFE layer 28 by, e.g., a method such as by a spraymechanism 42 to the upper surface 38 of the PTFE layer 28. In such anembodiment, the stretching agent 40 is applied to the PTFE layer 28after the PTFE layer 28 unwinds from spool 32 and passes under the spraymechanism 42. The stretching agent 40 may be applied uniformly over oneor both sides of the PTFE layer 28, on only one side of the PTFE layer28, or only on selected portions of the PTFE layer 28 at a temperatureof typically about 70° F. to about 135° F., specifically, about 105° F.to about 125° F., and more specifically, about 110° F. to about 120° F.

If a PTFE layer having low or substantially no fluid permeability isdesired, the PTFE layer 28 may be stretched in one or more directionswhile fully saturated until the desired thickness is achieved. It shouldbe noted that as the PTFE layer 28 is stretched, the capacity of theresulting stretched PTFE layer 36 to absorb stretching agent 40increases. As such, if it is desirable to maintain a saturated status ofthe PTFE layer 28 and stretched PTFE layer 36, it may be necessary toadd stretching agent multiple times or over a large area in order tomaintain that saturated state of the PTFE layer 36 and the effect oflubricant temperature for a period of time.

FIG. 4 illustrates the stretching agent or lubricant 40 being applied toupper surface 38 of the PTFE layer 28 by spray mechanism 42 as the PTFElayer 28 is being stretched transversely. For saturated stretchingembodiments, it may be necessary to apply sufficient stretching agent soas to pool or puddle the stretching agent on the upper surface 38 of thePTFE layer 28. The pooled or puddled stretching agent may be spread overthe upper surface 38 of the PTFE layer 28 by a skimming member 44 thathas a smooth contact edge 46 adjacent the upper surface 38 of the PTFElayer 28. While not shown, multiple skimming members may be used withsome or all having a smooth contact edge or alternatively agrooved/patterned contact edge. The skimming member 44 is disposedadjacent the spray mechanism 42 displaced from the spray mechanism inthe machine direction of the PTFE layer 28 such that the stretchingagent 40 applied by the spray mechanism 42 runs into the skimming member44 and is spread by the motion of the stretching agent 40 and PTFE layer28 relative to the skimming member 44. The skimming member 44 may be incontact with the upper surface 38 of the PTFE layer 28 or also may bedisposed slightly above the upper surface 38, depending on the desiredconfiguration of the setup, the type of stretching agent being used aswell as other factors.

Embodiments of methods discussed herein may be useful to reduce athickness of the PTFE layer 28 to a stretched PTFE layer 36 of anythickness down to about 0.00005 inch; typically from about 0.00005 inchand 0.005 inch. Typical transverse stretch ratios may be from about 3:1to about 20:1. In one embodiment, a calendered PTFE layer 28 having awidth of about 3 inches to about 6 inches, may be transverselystretched, as shown in FIGS. 3 and 4, into a stretched PTFE layer 36having a width of about 20 inches to about 60 inches. This represents astretch ratio of about 3:1 to about 12:1. In another embodiment, acalendered PTFE layer 28 having a width of about 3.5 inches to about 4.5inches may be transversely stretched, as shown in FIGS. 3 and 4, into astretched PTFE layer 36 having a width of about 20 inches to about 60inches. This represents a stretch ratio of about 7.8:1 to about 13:1.

As discussed above, the thickness, fluid permeability, porosity andaverage pore size of the PTFE layers 36 may be effected by the amountand temperature of stretching agent 40 applied to the layer 36 prior toor during stretching, the temperature of the layer, the stretching agentthat is applied to the PTFE layer, or both, prior to stretching and thestretch rate. By adjusting these parameters, these characteristics maybe optimized in order to produce a PTFE layer that is suited to aparticular application. For example, if the PTFE layer 36 is used as amoisture barrier for clothing, the parameters may be adjusted to producean average pore size of less than about 6 microns. Alternatively, if thePTFE layer 36 is used in an endovascular graft that benefits from tissuein-growth, the average pore size is adjusted to be greater than 6.0microns. In other embodiments, where the PTFE layer 36 is a barrierlayer for use in an endovascular graft, the pore size may be smaller,such as from about 0.01 micron to about 5.0 microns. In addition,embodiments of the stretched PTFE layer 36 are fusible and deformableand easily may be fused with or secured to other PTFE layers havingdifferent properties. At any point after the PTFE layer 28 is stretched,the stretched PTFE layer 36 may be sintered to amorphously lock themicrostructure of the PTFE layer 36. Sintering may be performed tocombine the stretched PTFE layer 36 with other layers of PTFE to formmulti-layer composite films, such as those used for endovascular graftsand the like discussed below.

The stretched PTFE layer optionally may be subjected to a secondstretching process, as shown in FIGS. 3, 4, 5 and 6, wherein thestretched PTFE layer 36 is formed into a twice-stretched PTFE layer 46.Once again, as discussed above, it is important to note that althoughthe method embodiments discussed herein are directed to a firsttransverse stretch and subsequently to a longitudinal or machinedirection stretch, the order of the stretch directions may be reversedand other combinations of stretch directions and numbers are alsocontemplated. For example, PTFE layer 28 may be stretched twice in themachine or longitudinal direction without any transverse stretching.PTFE layer 28 may be stretched first in a longitudinal or machinedirection and then in a transverse direction. In addition, a PTFE layer28 may be stretched three or more times. Some or all of the speeds,stretch ratios, temperatures, lubricant parameters and the likediscussed herein may be the same but need not necessarily and typicallywill not be the same for any of these various stretching stepsregardless of the order the stretching steps.

This optional second stretching process subjects the PTFE layer 36 toyet another mechanical working. The second stretching process shown inFIGS. 5 and 6 is being carried out in the machine direction; however,the second stretching process may also be carried out in any othersuitable direction, such as transversely. The twice-stretched PTFE layer46 is wound onto spool 48 after undergoing the second stretchingprocess. Additional stretching agent 40 optionally may be applied to asurface of the stretched PTFE layer 36 as the layer 36 is beingstretched a second time. If higher porosity and fluid permeability aredesired, the second stretch may be performed with the stretched layer 36in a dry state without the addition of lubricant during the secondstretch. If the stretched PTFE layer 36 has residual lubricant withoutadditional lubricant added, the second stretching process will generatea microstructure having significant nodes connected by fibrils. Thesecond stretching process may be carried out at a temperature of about85° F. to about 95° F. for some embodiments. The stretch ratio for thesecond stretch maybe up to about 20:1, specifically, about 6:1 to about10:1.

If the PTFE layer 28 is stretched in two or more directions, the rate ofstretching in the two directions; e.g., the machine direction and theoff-axis or transverse direction, may have different or the same stretchrates. For example, when the PTFE layer 28 is being stretched in themachine direction (e.g., first direction), the rate of stretching istypically in the range from about two percent to about 100 percent persecond; specifically, from about four percent to about 20 percent persecond, and more specifically about five percent to about ten percentper second. In contrast, when stretching in the cross machine ortransverse direction, the rate of stretching may be in the range fromabout one percent to about 300 percent per second, specifically fromabout ten percent to about 100 percent per second, and more specificallyabout 15 percent to about 25 percent per second.

Stretching in the different directions may be carried out at the sametemperatures or at different temperatures. For example, stretching inthe machine direction is generally carried out at a temperature belowabout 572° F., and for some embodiments, below about 239° F. Incontrast, stretching in the transverse direction is typically carriedout at a temperature above the glass transition temperature, and usuallyfrom about 80° F. to about 100° F. Stretching PTFE layers 28 at lowertemperatures will reduce stretching agent 40 evaporation and retain thestretching agent 40 in the PTFE layer 28 for a longer period of timeduring processing.

Either the stretched PTFE layer 36 or the twice-stretched PTFE layer 46optionally may be calendered in order to further thin and densify thematerial. The twice-stretched PTFE layer 46 is shown being calendered inFIGS. 7 and 8. In this example, the twice-stretched PTFE layer 46 isunwound from spool 48, passed through calender rollers 50 and 52, formedinto a densified layer 54, then taken up on spool 54. The calendermachine may be the same machine or a different machine as that indicatedin FIG. 2 and discussed above. This final calendering or densificationof PTFE layer 46 generally produces a highly densified PTFE layer 54that has no discernable microstructure features, such as pores, and haslow or substantially no fluid permeability. The methods of compressingand stretching PTFE layers may both be used to control thinning of thePTFE layer and the microstructure that results from the thinningprocess. The densified PTFE layer 54 may also lack the suppleness andlimpness mechanical properties of the stretched PTFE layers 36 and 46discussed above. The rollers 50 and 52 may be adjusted to have anysuitable separation to produce a PTFE layer 54 having a thickness ofabout 0.00005 inch to about 0.005 inch. The rollers 50 and 52 may alsobe heated during the calendering process, with typical temperaturesbeing from about 90° F. to about 250° F.; specifically, from about 120°F. to about 160° F.; more specifically, from about 130° F. to about 150°F.

The following example describes specific methods of manufacturing of thestretched PTFE layers 36. In this embodiment, 1000 grams of resin arecompounded with an isoparaffin based lubricant; specifically, ISOPAR® M,in a mass ratio of lubricant-to-PTFE compound from about 15% to about25%. Compounding of the PTFE resin and lubricant is carried out at atemperature below 50° F., which is well below the glass transitiontemperature of the PTFE resin of between about 57° F. to about 75° F.

The PTFE compound 10 maybe formed into a billet and stored at atemperature of about 105° F. to about 125° F. for six or more hours toensure that the lubricant substantially has penetrated and absorbedthrough the resin. Thereafter, the PTFE compound 10 is placed in anextruder 12, as shown in FIG. 1. The PTFE compound 10 may then be pasteextruded from the orifice 22 of the die 16 of the extruder 12 at atemperature above the resin glass transition temperature. In oneembodiment, the paste is extruded at a temperature from about 80° F. to120° F. A reduction ratio, e.g., a ratio of a cross sectional area ofthe PTFE compound 10 before extrusion to the cross section area of thePTFE extrudate 24 after extrusion, may be from about 10:1 to about400:1, and specifically may be from about 80:1 to about 120:1. Theextruder 12 maybe a horizontal extruder or a vertical extruder. Theorifice 22 of the extrusion die 16 determines the final cross sectionalconfiguration of the extruded PTFE ribbon 24. The orifice 22 shape orconfiguration of the extrusion die 16 may be tubular, square,rectangular or any other suitable profile. It may be desirable topreform the PTFE compound (resin and lubricant) into a billet.

The PTFE extrudate ribbon 24 is then calendered, as shown in FIG. 2, ata temperature from about 100° F. to about 160° F. to reduce a thicknessof the PTFE ribbon 24 and form a PTFE layer or film 28. The temperatureat calendering may be controlled by controlling the temperature of therollers 30 of the calender machine. The PTFE layer may be calendereddown to a thickness from about 0.001 inch to about 0.006 inch, andspecifically, down to a thickness of about 0.002 inch to about 0.003inch. At the end of the calendering, the calendered PTFE layer 28 mayhave a lubricant content of about 10% by weight to about 20% by weight.

Referring again to FIGS. 3 and 4, after calendering, one side or bothsides of the calendered PTFE layer 28 are sprayed with anisoparaffin-based stretching agent 40 at a prescribed temperature sothat the PTFE film or layer 28 is flooded and fully saturated throughthe thickness of the PTFE layer 28. The saturated, calendered PTFE layermay then be stretched in a direction that is substantially orthogonal tothe calendering direction by a tentering machine 34 to reduce athickness of the PTFE layer 28 and form a stretched PTFE layer 36. Thestretched PTFE layer 36 may have a thickness of about 0.00005 inch toabout 0.005 inch; specifically, the stretched PTFE layer 36 may have athickness of about 0.0002 inch to about 0.002 inch. The PTFE layer 28typically is tentered or stretched at an elevated temperature above theglass transition temperature, specifically, from about 80° F. to about100° F., more specifically, about 85° F. to about 95° F.

Wet tentering with the stretching agent 40 allows the PTFE layer 28 tobe thinned without creating substantial porosity and fluid permeabilityin the stretched PTFE layer 36. While the stretched PTFE layer 36 willhave a porosity, its porosity and pore size typically will not be largeenough to be permeable to liquids, and often will be small enough tohave substantially no fluid permeability. In addition, the stretchedPTFE layer embodiment 36 does not have the conventional node and fibrilmicrostructure but instead has a closed cell microstructure in whichboundaries of adjacent nodes are directly connected with each other. Thefluid-impermeable stretched PTFE film or layer 36 typically may have adensity from about 0.5 g/cm³ to about 1.5 g/cm³, but it may have alarger or smaller density for some embodiments. In addition, with regardto all of the methods of processing layers of PTFE discussed above, anyof the PTFE layers produced by these methods may also be sintered at anypoint in the above processes in order to substantially fix themicrostructure of the PTFE layer. A typical sintering process may be toexpose the PTFE layer to a temperature of about 350° C. to about 380° C.for several minutes; specifically, about 2 minutes to about 5 minutes.

The various methods discussed above may be used to produce PTFE layershaving a variety of desirable properties. The scanning electronmicroscope (SEM) images shown in FIGS. 9 to 13 illustrate differentmagnifications of a microstructure of a PTFE film or layer 110 made inaccordance with embodiments of the present invention. PTFE layer 110 hasa generally closed cell microstructure 112 that is substantially free ofthe conventional node and fibril microstructure commonly seen inexpanded PTFE layers. Embodiments of the PTFE film 110 may have lowfluid-permeability, or no or substantially no fluid-permeability. One ormore of PTFE layer 110 may be used as a barrier layer to prevent a fluidsuch as a liquid or gas from permeating or escaping therethrough.

At a magnification of 20,000, as seen in FIG. 9, the microstructure ofthe stretched PTFE layer 110 resembles a pocked-like structure thatcomprises interconnected high density regions 114 and pockets or pores116 between some of the high density regions 114. The PTFE film 110 maybe considered to have a closed cell network structure withinterconnected strands connecting high density regions 114 in which ahigh density region grain boundary is directly connected to a grainboundary of an adjacent high density region. Unlike conventionalexpanded PTFE (“ePTFE”) which typically has a substantial node andfibril microstructure that is discernable when viewed at a SEMmagnification of 20,000, PTFE layer 110 lacks the distinct, parallelfibrils that interconnect adjacent nodes of ePTFE and has no discernablenode and fibril microstructure when viewed at a SEM magnification of20,000, as shown in FIG. 9. The closed cell microstructure of the PTFElayer 110 provides a layer having low or substantially no fluidpermeability that may be used as “a barrier layer” to prevent liquidfrom passing from one side of the PTFE layer to the opposite side.

Though PTFE film or layer 110 is configured to have low or substantiallyno fluid permeability, PTFE layer 110 nonetheless has a porosity. ThePTFE layer 110 typically has an average porosity from about 20% to about80%, and specifically from about 30% and about 70%. In one embodiment, aPTFE film 110 has a porosity of about 30% to about 40%. In anotherembodiment, a PTFE layer 110 has a porosity of about 60% to about 70%.Porosity as described in these figures is meant to indicate the volumeof solid PTFE material as a percentage of the total volume of the PTFEfilm 110. An average pore size in the PTFE layer 110 is may be less thanabout 20 microns, and specifically less than about 0.5 micron. In oneembodiment, a PTFE layer 110 has an average pore size of from about 0.01micron to about 0.5 micron. As can be appreciated, if tissue ingrowth isdesired, the PTFE film 110 may have an average pore size of greater thanabout 6.0 microns. As described below, depending on the desiredproperties of the resultant PTFE layer 110, embodiments of methods maybe modified so as to vary the average porosity and average pore size ofthe PTFE film 110 in a continuum from 10 microns to 50 microns down tosubstantially less than about 0.1 micron.

PTFE layer 110 may have a density from about 0.5 g/cm³ to about 1.5g/cm³, and specifically from about 0.6 g/cm³ to about 1.5 g/cm³. Whilethe density of the PTFE film 110 is typically less than a density for afully densified PTFE layer (e.g., 2.1 g/cm³), if desired, the density ofthe PTFE layer 110 may be densified to a higher density level so thatthe density of the PTFE layer 110 is comparable to a fully densifiedPTFE layer. FIGS. 9 to 13 illustrate a PTFE film 110 having a closedmicrostructural network and that is substantially impermeable to liquidand gas; other embodiments of PTFE layers may be manufactured using themethods discussed herein to have other suitable permeability values andpore sizes.

PTFE film 110 may have an average thickness that is less than about0.005 inch, specifically from about 0.00005 inch to about 0.005 inch,and more specifically from about 0.0001 inch to about 0.002 inch.

While embodiments of methods discussed herein are directed tomanufacturing PTFE layers, it should be appreciated that the methodsdiscussed may also be useful in the manufacture of otherfluoropolymer-based films having substantial, low or substantially nofluid permeability. As such, the methods discussed herein are notlimited to the processing of PTFE materials. For example, the processingof other fluoropolymer resin-based materials, such as copolymers oftetrafluororethylene and other monomers, is also contemplated.

The PTFE layers and PTFE films may be used in a variety of ways. Forexample, the PTFE layer and PTFE film embodiments of the presentinvention may be used for prosthetic devices such as a vascular graft,breast implants and the like. Other applications include tubing,protective clothing, insulation, sports equipment, filters, membranes,fuel cells, ionic exchange barriers, gaskets as well as others.Referring now to FIG. 14, PTFE layer 110 may be combined with, bondedto, or otherwise coupled, affixed or attached, partially or completely,to at least one additional layer 118 to form a composite film 120.Depending on the use of composite film 120, layer 118 may be chosen tohave properties that combine with the properties of layer 110 to givethe desired properties in composite film 120. The additional layer 118may include a porous PTFE layer, a substantially non-porous PTFE layer,an air or liquid permeable PTFE layer, an air- or liquid-impermeablelayer, an ePTFE layer, a non-expanded PTFE layer, a fluoropolymer layer,a non-fluoropolymer layer, or any combination thereof. In oneembodiment, layer 118 is a porous, fluid permeable, expanded PTFE layerhaving a conventional node and fibril microstructure. If desired, one ormore reinforcing layers (not shown) optionally may be coupled to thecomposite PTFE film 120. The reinforcing layer may be disposed betweenlayers 110 or 118, or the reinforcing layer(s) may be coupled to anexposed surface of PTFE layer 110, PTFE layer 118, or both. PTFE layer110 and layer 118 may be combined, bonded to, or otherwise coupled,affixed or attached, partially or completely, to one another using anysuitable method known in the art. For example, an adhesive may be usedto selectively bond at least a portion of layers 110 and 118 to eachother. Alternatively, heat fusion, pressure bonding, sintering, and thelike may be used to bond at least a portion of layers 110 and 118 toeach other.

FIGS. 15 and 16 are transverse cross-sectional views of two compositetubular structures 130 and 140, respectively. Tubular structures 130 and140 may be a portion or section of an endovascular graft or the like. Asshown in FIG. 15, tubular structure 130 includes an inner tubular body132 that comprises an inner surface 134 and an outer surface 136.Tubular body 132 may comprise one or more layers of fluid-permeablePTFE. Such a fluid-permeable layer of PTFE may have a Gurley measurementof less than about 10 Gurley seconds. Tubular structure 130 furthercomprises an outer tubular body 138 that comprises an inner surface 137and an outer surface 139. Inner surface 137 of outer tubular body 138 iscoupled to the outer surface 136 of the inner tubular body 132. Tubularbody 138 may comprise one or more PTFE layers having lowfluid-permeability or substantially no fluid-permeability. In thisconfiguration, inner surface 134 of the tubular body 132 defines aninner lumen 135 of tubular structure 130 and the outer surface 139 ofthe tubular body 138 defines an outer surface 139 of the tubularstructure 130. Tubular body 138 may be combined, bonded to, or otherwisecoupled, affixed or attached, partially or completely, to the tubularbody 132 through any suitable method known in the art. For example, anadhesive may be used to selectively bond at least a portion of tubularbody 138 and tubular body 132 to each other. Alternatively, heat fusion,pressure bonding, sintering, and the like, or any combination thereof,may be used to bond at least a portion of tubular body 138 and tubularbody 132 to each other.

As shown in FIG. 16, tubular structure 140 includes an inner tubularbody 142 that comprises an inner surface 144 and an outer surface 146.Tubular body 142 may comprise one or more layers of PTFE having low orsubstantially no fluid permeability. Tubular structure 140 furthercomprises an outer tubular body 148 that comprises an inner surface 147and an outer surface 149. Inner surface 147 of outer tubular body 148 iscoupled to the outer surface 146 of the inner tubular body 142. Outertubular body 148 may comprise one or more layers of fluid-permeablePTFE. Embodiments of fluid-permeable layers of PTFE may have a Gurleymeasurement of less than about 10 Gurley seconds. In this configuration,inner surface 144 of the inner tubular body 142 defines an inner lumen145 of tubular structure 140 and the outer surface 149 of the outertubular body 148 defines an outer surface 149 of the tubular structure140. Tubular body 148 may be combined, bonded to, or otherwise coupled,affixed or attached, partially or completely, to the tubular body 142through any suitable method known in the art. For example, an adhesivemay be used to selectively bond at least a portion of tubular body 148and tubular body 132 to each other. Alternatively, heat fusion, pressurebonding, sintering, and the like, or any combination thereof, may beused to bond at least a portion of tubular body 148 and tubular body 142to each other.

Tubular structures 130 or 140 may define an inner diameter ID which isthe diameter of the inner surface, which may define the area of flowthrough tubular structure 130 or 140. An outer diameter OD, which is thediameter of the outer surface 139 or 149 of the outer tubular layer 138or 148. The inner diameter ID and outer diameter OD may be any desireddiameter. For use in an endovascular graft, the inner diameter ID but istypically from about 10 mm to about 40 mm and the outer diameter OD istypically from about 12 mm to about 42 mm. The tubular layers may haveany suitable thickness, however, fluid-impermeable PTFE layers 138 and142 have a thickness from about 0.0005 inch and about 0.01 inch thick,and specifically from about 0.0002 inch to about 0.001 inch. Similarly,fluid-permeable PTFE layers 132 or 148 may also be any thicknessdesired, but typically have a thickness from about 0.0001 inch and about0.01 inch, and specifically from about 0.0002 inch to about 0.001 inch.As can be appreciated, the thicknesses and diameters of the tubularstructures 130 or 140 will vary depending on the use of the tubularstructures.

Tubular structures 130 or 140 may be formed as tubes throughconventional tubular extrusion processes. Typically, however, tubularstructures 130 or 140 may be formed from PTFE layers 110 or 118, asshown in FIG. 14, that are folded on a shape forming mandrel over eachother so that ends of the layers are overlapped and bonded (not shown).As another alternative, PTFE layers 110 or 118 may be helically woundabout the shape forming mandrel to form the tubular structure. Someexemplary methods of forming a tubular PTFE structure is described incommonly owned U.S. patent application Ser. Nos. 10/029,557 (whichpublished as U.S. Patent Application No. 2003/0116260 A1) and entitled“Methods and Apparatus for Manufacturing an Endovascular Graft Section”,10/029,584 (which published as U.S. Pat. No. 7,090,693) and entitled“Endovascular Graft Joint and Method of Manufacture”, both filed on Dec.20, 2001 to Chobotov et al., and U.S. Pat. No. 6,776,604 to Chobotov etal., the complete disclosures of which are incorporated herein byreference.

The films and layers discussed herein are not limited to a single porousPTFE layer 118 and a single PTFE layer or film 110 having low orsubstantially no fluid permeability. The composite films 120 and tubularstructures 130 or 140 may include a plurality of porous fluid permeablePTFE layers (having the same or different node and fibril size andorientation, porosity, pore size, and the like), one or more non-porous,densified PTFE layers, and/or one or more PTFE layers 110 having low orsubstantially no fluid permeability. For example, PTFE layer 110 havinglow or substantially no fluid permeability may be disposed between aninner and outer porous PTFE film or layer. The inner and outer porousPTFE layers may have varying porosities or the same porosities. In suchembodiments, the PTFE layer 110 may have a reduced thickness relative tothe porous PTFE layers. In other embodiments, however, the PTFE layer110 may have the same thickness or larger thickness than the porous PTFElayers. As an alternative embodiment to FIGS. 15 and 16, tubularstructures 130 or 140 may comprise inner and outer tubular bodies thatboth have low or substantially no fluid permeability.

Referring now to FIG. 17, a tubular structure that is in the form of aninflatable endovascular graft 50 is shown. For the purposes of thisapplication, with reference to endovascular graft devices, the term“proximal” describes the end of the graft that will be oriented towardsthe oncoming flow of bodily fluid, typically blood, when the device isdeployed within a body passageway. The term “distal” therefore describesthe graft end opposite the proximal end. Graft 150 has a proximal end151 and a distal end 152 and includes a generally tubular structure orgraft body section 153 comprised of one or more layers of fusiblematerial, including such materials as PTFE and ePTFE. The inner surfaceof the tubular structure defines an inner diameter and acts as a luminalsurface for flow of fluids therethrough. The outer surface of thetubular structure defines an abluminal surface that is adapted to bepositioned adjacent the body lumen wall, within the weakened portion ofthe body lumen, or both. Note that although FIG. 17 shows an inflatableendovascular graft, the layers and films of the present invention may beused in non-inflatable endovascular grafts as well, in addition to othermedical and non-medical applications.

A proximal inflatable cuff 156 may be disposed at or near a proximal end151 of graft body section 153 and a distal inflatable cuff 157 may bedisposed at or near a graft body section distal end 152. Graft bodysection 153 forms a longitudinal lumen that is configured to confine aflow of fluid, such as blood, therethrough. Graft 150 may bemanufactured to have any desired length and internal and externaldiameter but typically ranges in length from about 5 cm to about 30 cm;specifically from about 10 cm to about 30 cm. If desired, a stent 159may be attached at the proximal end 151 and/or the distal end 152 of thegraft 150. Depending on the construction of the cuffs 156 and 157 andgraft body section 153, inflation of cuffs 156 and 157, when notconstrained (such as, e.g., by a vessel or other body lumen), may causethe cuffs 156 and 157 to assume a generally annular or toroidal shapewith a generally semicircular longitudinal cross-section. Inflatablecuffs 156 and 157 may be designed to generally, however, conform to theshape of the vessel within which it is deployed. When fully inflated,cuffs 156 and 157 may have an outside diameter ranging from about 10 mmto about 45 mm; specifically from about 16 mm to about 42 mm.

At least one inflatable channel 158 may be disposed between and in fluidcommunication with proximal inflatable cuff 156 and optional distalinflatable cuff 157. Inflatable channel 158 in the FIG. 17 example has ahelical configuration and provides structural support to graft bodysection 153 when inflated to contain an inflation medium. Inflatablechannel 158 further prevents kinking and twisting of the tubularstructure or graft body section when it is deployed within angled ortortuous anatomies as well as during remodeling of body passageways,such as the aorta and iliac arteries, within which graft 150 may bedeployed. Together with proximal and distal cuffs 156 and 157,inflatable channel 158 forms an inflatable network over the length ofthe body 153. Depending on the desired characteristics of theendovascular graft 150, at least one layer of the graft may be a PTFElayer having low or substantially no fluid permeability such as PTFElayer or film 110. The PTFE layer may be one of the layers that formsthe inflatable channels 158, or the PTFE layer may surround or beunderneath the inflatable channel 158 and cuffs 156 and 157.

Graft body 153 may be formed of two or more layers or strips of PTFEthat are selectively fused or otherwise adhered together as describedherein, to form the inflatable cuffs 156 and 157 and inflatable channel158 therebetween. A detailed description of some methods ofmanufacturing a multi-layered graft are described in commonly owned U.S.patent application Ser. Nos. 10/029,557 (which published as U.S. PatentApplication Publication No. 2003/0116260 A1), 10/029,584 (whichpublished as U.S. Pat. No. 7,090,693), U.S. patent application Ser. No.10/168,053 (which published as U.S. Pat. No. 8,226,708), filed Jun. 14,2002 and entitled “Inflatable Intraluminal Graft” to Murch, and U.S.Pat. No. 6,776,604 to Chobotov et al., the complete disclosures of whichare incorporated herein by reference.

FIGS. 18 to 21 illustrate transverse cross sectional views of differentembodiments of inflatable channel 158. As can be appreciated, theembodiments of FIGS. 18 to 21 may also be applicable to the proximal anddistal cuffs 156 and 157. Inflatable channel 158 defines an inflatablespace 162 that is created between an inner layer 164 and outer layer166. If desired an inflation medium 167 may be delivered into the space162 to inflate inflatable space 162. Inflation medium 167 optionally mayinclude a deliverable agent 168 as shown in FIGS. 18 to 21, such as atherapeutic agent 168 that may be configured to be diffused in acontrolled manner or otherwise transmitted through pores (not shown) ininner layer 164, outer layer 166 or both. The embodiments shown in FIGS.18-21 are merely exemplary, as it may be desirable to have preferentialdiffusion of the deliverable agent 168 through layer 164 or layer 166.In addition, both layers 164 and 166 may be configured to allow asignificant amount of diffusion of deliverable agent 168, but with oneof the two layers having a greater permeability to the deliverable agent168 than the other layer. While inner layer 164 and layer 166 are shownas having only a single layer of material, it should be appreciated thateach of layers 164 or 166 may include one or more layers to form acomposite film of fluid-permeable PTFE, PTFE having low fluidpermeability, PTFE having substantially no fluid permeability or anycombination thereof. A more complete description of methods and devicesfor the delivery of a therapeutic agent can be found in commonly ownedU.S. patent application Ser. No. 10/769,532 (which published as U.S.Patent Application Publication No. 2005/0171593 A1), filed Jan. 30, 2004and entitled “Inflatable Porous Implants and Methods for Drug Delivery”to Whirley et al., the complete disclosure of which is incorporatedherein by reference. A description of exemplary inflation mediummaterials can be found in commonly owned U.S. patent application Ser.No. 11/097,467 (which published as U.S. Patent Application PublicationNo. 2006-0222596 A1), filed Apr. 1, 2005 and entitled “A Non-Degradable,Low Swelling, Water Soluble, Radiopaque Hydrogel” to Askari et al., thecomplete disclosure of which is incorporated herein by reference.

In the embodiment shown in FIG. 18, outer layer 166 is permeable tofluids so as to allow the therapeutic agent 168, which may be a liquid,to diffuse over time in the direction of arrow 169 through outer layer166. In such embodiments, inner layer 164 typically has a low orsubstantially no fluid permeability, and could therefore be considered a“barrier layer.” Because the inner “barrier” layer 164 has low orsubstantially no fluid permeability and outer layer 166 is fluidpermeable, the therapeutic agent will preferentially diffuse from space162 in the direction of arrow 169. The use of one (or more) porous fluidpermeable outer PTFE layers and an inner layer 164 having low orsubstantially no fluid permeability provides for improved release of atherapeutic agent through liquid permeable outer layer 166. Varying theporosity or pore size across at least a portion of outer layer 166 mayprovide even more localized delivery of the therapeutic agent 168through outer layer 166.

In an alternative configuration shown in FIG. 19, inner layer 164 may besubstantially fluid-permeable to allow the therapeutic agent 168 toselectively diffuse in the direction of arrow 169 through inner layer164 and into the lumen of the tubular structure (e.g., lumen 135, 145 ofFIGS. 15 and 16). In such embodiments, outer layer 166 typically has noor substantially no fluid-permeability and acts as a “barrier layer.” Assuch, the therapeutic agent will preferentially diffuse from space 162in the direction of arrow 169. The use of porous fluid permeable PTFElayers and outer layer 166 having low or substantially no fluidpermeability provides for improved release of a therapeutic agent intothe inner lumen through fluid permeable inner layer 164. Varying thepermeability and/or porosity or pore size across at least a portion ofinner layer 164 may provide even more localized delivery of thetherapeutic agent 168 through layer 164.

As shown in FIG. 20, if it is desired to prevent the inflation medium167 from escaping from inflatable space 162, both the inner layer 164and outer layer 166 may comprise a “barrier” layer having low orsubstantially no fluid permeability. In such embodiments, the inner andouter layers 164 and 166 have low or substantially no fluidpermeability. In such embodiments, inflation material 167 typically willnot contain a therapeutic agent. Referring to FIG. 21, the inflatablechannel may be a substantially tubular channel 170 that is fused orotherwise adhered to layer 164 that defines a portion of the graft. Ifdelivery of a therapeutic agent is desired, tubular channel 170 will beliquid-permeable and will allow diffusion of the therapeutic agent 168through pores in tubular channel 170. In some embodiments, by varyingthe permeability and/or porosity or pore size across at least a portionof channel 170 may provide a localized delivery of the therapeutic agent168 selected portions of channel 170. If however, it is desired toprevent the inflation fluid 167 from escaping from inflatable space 162,then tubular channel 170 will act as a barrier layer and may comprise atleast one layer of PTFE having low or substantially no fluidpermeability.

Referring now to FIGS. 22 and 23, the respective graft embodiments 150and 180 shown include an inflatable channel 158 has portions with acircumferential configuration as opposed to the helical configuration ofthe inflatable channel 158 shown in FIG. 17. The circumferentialconfiguration of portions of the inflatable channel 158 may beparticularly effective in providing the needed kink resistance forendovascular graft for effectively treating diseased body passagewayssuch as a thoracic aortic aneurysm (TAA), abdominal aortic aneurysm(AAA), in which highly angled and tortuous anatomies are frequentlyfound. In alternative embodiments, other cuff and channel configurationsare possible. Inflatable channel 158 may be configured circumferentiallyas shown in FIGS. 22 and 23.

In addition to the substantially tubular grafts of FIG. 22, bifurcatedendovascular grafts as shown in FIG. 23, are also contemplated. Thebifurcated endovascular graft 180 may be utilized to repair a diseasedlumen at or near a bifurcation within the vessel, such as, for example,in the case of an abdominal aortic aneurysm in which the aneurysm to betreated may extend into the anatomical bifurcation or even into one orboth of the iliac arteries distal to the bifurcation. In the followingdiscussion, the various features of the graft embodiments previouslydiscussed may be used as necessary in the bifurcated graft 80 embodimentunless specifically mentioned otherwise.

Graft 180 comprises a first bifurcated portion 182, a second bifurcatedportion 184 and main body portion 186. The size and angular orientationof the bifurcated portions 182 and 184 may vary to accommodate graftdelivery system requirements and various clinical demands. The size andangular orientation may vary even between portion 182 and 184. Forinstance, each bifurcated portion or leg is shown in FIG. 23 tooptionally have a different length. First and second bifurcated portions182 and 184 are generally configured to have an outer inflated diameterthat is compatible with the inner diameter of a patient's iliacarteries. First and second bifurcated portions 182 and 184 may also beformed in a curved shape to better accommodate curved and even tortuousanatomies in some applications. Together, main body portion 186 andfirst and second bifurcated portions 182 and 184 form a continuousbifurcated lumen, similar to the inner lumens of FIG. 22, which isconfigured to confine a flow of fluid therethrough. A completedescription of some desirable sizes and spacing of inflatable channelsmay be found in commonly owned U.S. patent application Ser. No.10/384,103 (which published as U.S. Patent Application Publication No.20040176836 A1), entitled “Kink-Resistant Endovascular Graft” and filedMar. 6, 2003 to Kari et al., the complete disclosure of which isincorporated herein by reference.

While not shown, it should be appreciated, that instead ofcircumferential channels and longitudinal channels, the bifurcated graft180 may comprise a helical inflatable channel 158, similar to that ofthe graft embodiment shown in FIG. 17 (or other channel geometries toachieve desired results), or a combination of helical andcircumferential channels. A complete description of some embodiments ofendovascular grafts that have helical and cylindrical channelconfigurations may be found in co-pending and commonly owned U.S. patentapplication Ser. No. 10/384,103 (which published as U.S. PatentApplication Publication No. 20040176836 A). Other endovascular graftsthat the liquid-impermeable PTFE film may be used with are described inU.S. Pat. No. 6,395,019 to Chobotov, U.S. Pat. No. 6,132,457 toChobotov, U.S. Pat. No. 6,331,191 to Chobotov, and U.S. patentapplication Ser. Nos. 10/327,711 (which published as U.S. PatentApplication Publication No. 2003/0125797 A1), entitled “AdvancedEndovascular Graft” to Chobotov et al. and filed Dec. 20, 2002,10/168,053 (which published as U.S. Pat. No. 8,226,708), the completedisclosures of which are incorporated herein by reference.

As can be appreciated, the inflatable portions of the graft 180optionally may be configured to have varying levels of fluidpermeability and/or porosity, either within or between particular cuffs,channels or cuff/channel segments, so as to provide for controlled drugdelivery, programmed drug delivery or both, into the vessel wall orlumen of the graft via elution of the agent from pores in the layers.For example, any desired portion of the graft 180 may include PTFElayers having low or substantially no fluid permeability. Such aconfiguration would be useful in applications in which the drug deliveryrate and other properties of the graft or stent-graft (e.g. mechanicalproperties) may be selected for the particular clinical needs andindication that is contemplated for that device. In addition, the fluidpermeability and/or porosity may be uniform within a particular cuff orchannel but different between any given channel and/or cuffs. Inaddition to improved drug delivery, the variable porosity of the outersurface of the graft may also be beneficial for promoting tissuein-growth into the graft. It may be possible to make portions of thegraft that are in direct contact with the body lumen to have a higherporosity and/or larger pore size so as to promote tissue in-growth. Inparticular, tissue in-growth may be beneficial adjacent to the proximaland distal ends of the graft.

With regard to the above detailed description, like reference numeralsused therein refer to like elements that may have the same or similardimensions, materials and configurations. While particular forms ofembodiments have been illustrated and described, it will be apparentthat various modifications can be made without departing from the spiritand scope of the embodiments of the invention. Accordingly, it is notintended that the invention be limited by the forgoing detaileddescription.

The following listing of embodiments are useful with the presentinvention:

Embodiment 1. A method of processing PTFE, comprising:

-   -   providing a layer of PTFE;    -   applying a stretching agent to at least a portion of the layer        of PTFE; and    -   stretching the layer of PTFE while the layer of PTFE is wet with        the stretching agent to form a stretched layer of PTFE.

Embodiment 2. The method of embodiment 1 wherein the stretching agent isapplied to substantially all of the layer of PTFE prior to stretching.

Embodiment 3. The method of embodiment 1 wherein stretching the layer ofPTFE comprises stretching the layer of PTFE by a stretch ratio of about2:1 to about 20:1.

Embodiment 4. The method of embodiment 1 wherein the stretching of thelayer of PTFE comprises stretching in a machine direction.

Embodiment 5. The method of embodiment 1 wherein the stretching of thelayer comprises stretching the layer of PTFE in a direction transverseto the machine direction.

Embodiment 6. The method of embodiment 1 further comprising calenderingthe stretched layer of PTFE to densify and compress the layer of PTFE.

Embodiment 7. The method of embodiment 1 wherein the stretching agentcomprises an isoparaffin.

Embodiment 8. The method of embodiment 1 wherein the stretching agent isselected from the group consisting of naphtha, mineral spirits, alcohol,MEK, toluene and alcohol.

Embodiment 9. The method of embodiment 1 wherein a lubricant content ofthe layer of PTFE prior to application of the stretching agent is about0 percent by weight to about 22 percent by weight.

Embodiment 10. The method of embodiment 1 further comprising spreadingthe stretching agent after application to the layer of PTFE with askimming member disposed adjacent the layer of PTFE.

Embodiment 11. The method of embodiment 1 wherein stretching of thelayer of PTFE is performed at a temperature of about 80° F. to about100° F.

Embodiment 12. The method of embodiment 1 wherein stretching of thelayer of PTFE is performed at a temperature of about 85° F. to about 95°F.

Embodiment 13. The method of embodiment 1 wherein the stretching agentis applied to the layer of PTFE at a temperature of about 110° F. toabout 130° F.

Embodiment 14. The method of embodiment 13 wherein the stretching agentis applied to the layer of PTFE at a temperature of about 115° F. toabout 125° F.

Embodiment 15. The method of embodiment 1 wherein stretching the layerof PTFE is performed at a temperature that is just above the glasstransition temperature of the PTFE layer material.

Embodiment 16. The method of embodiment 1 wherein the provided layer ofPTFE is produced by extruding a compounded PTFE resin through anextruder to form a PTFE ribbon extrudate.

Embodiment 17. The method of embodiment 16 wherein the compounded resinis extruded to an extrudate having a ribbon configuration and having athickness of about 0.020 inch to about 0.040 inch.

Embodiment 18. The method of embodiment 16 wherein the PTFE ribbonextrudate is calendered to a reduced thickness of about 0.001 inch toabout 0.005 inch prior to stretching.

Embodiment 19. The method of embodiment 1 wherein the stretching agentis applied to the layer of PTFE in sufficient quantity such that atleast a portion of the layer of PTFE is saturated with stretching agentat the time of stretching.

Embodiment 20. The method of embodiment 1 further comprising stretchingthe stretched layer of PTFE a second time with no stretching agent addedto the stretched layer of PTFE during the second stretch.

Embodiment 21. The method of embodiment 20 wherein the stretched layerof PTFE has a sufficiently low stretching agent content so to form adiscernable node and fibril microstructure in the stretched layer ofPTFE during the second stretch.

Embodiment 22. The method of embodiment 1 further comprising sinteringthe stretched layer of PTFE.

Embodiment 23. A method of processing PTFE, comprising:

-   -   providing a layer of PTFE;    -   applying a stretching agent to at least a portion of the layer        of PTFE until a portion of the layer of PTFE is saturated with        the stretching agent to form a saturated portion; and    -   stretching the layer of PTFE while the saturated portion of the        layer of PTFE is saturated with the stretching agent.

Embodiment 24. The method of embodiment 23 wherein substantially all ofthe layer of PTFE is saturated with stretching agent to form a saturatedportion prior to stretching.

Embodiment 25. The method of embodiment 24 further comprising sinteringthe layer of PTFE.

Embodiment 26. A method of processing PTFE, comprising:

-   -   providing a stretched layer of PTFE that has been stretched in        at least a first direction;    -   applying a stretching agent to at least a portion of the        stretched layer of PTFE; and    -   stretching the stretched layer of PTFE while the stretched layer        of PTFE is wet with the stretching agent.

Embodiment 27. The method of embodiment 26 wherein the stretching agentis applied to substantially all of the stretched layer of PTFE prior tothe stretching of the stretched layer.

Embodiment 28. The method of embodiment 26 wherein the stretched layerof PTFE has a lubricant content of below about 3% by weight prior toapplication of the stretching agent.

Embodiment 29. The method of embodiment 26 wherein the stretched layerof PTFE has been stretched in the first direction with a sufficientlylow stretching agent content so to form a discernable node and fibrilmicrostructure in the stretched layer of PTFE during the stretch in thefirst direction.

Embodiment 30. The method of embodiment 26 wherein the stretching of thestretched layer is performed in a direction different than the firstdirection.

Embodiment 31. The method of embodiment 30 wherein the first directionis a machine direction and the stretching of the stretched layer isperformed in a transverse direction.

Embodiment 32. The method of embodiment 30 wherein the stretching of thestretched layer of PTFE material is performed in a transverse directionby a stretch ratio of about 2:1 to about 30:1.

Embodiment 33. The method of embodiment 26 wherein the stretching of thestretched layer is performed in substantially the same direction as thefirst direction.

Embodiment 34. The method of embodiment 33 wherein the first directionis a machine direction and the stretching of the stretched layer isperformed in the machine direction.

Embodiment 35. The method of embodiment 26 wherein the stretched layerof PTFE has been stretched in the first direction while the layer ofPTFE was wet with stretching agent applied to the layer of PTFE.

Embodiment 36. The method of embodiment 26 further comprising sinteringthe layer of PTFE.

Embodiment 37. A method of processing PTFE, comprising:

-   -   providing a layer of PTFE;    -   applying a stretching agent to at least a portion of the layer;    -   stretching the layer of PTFE while the layer of PTFE is wet with        the stretching agent to form a stretched layer of PTFE;    -   stretching the stretched layer of PTFE a second time; and    -   calendering the twice-stretched layer of PTFE so as to densify        and further thin the twice-stretched layer of PTFE.

Embodiment 38. The method of embodiment 37 wherein the stretching agentis applied to substantially all of the surface of the layer prior tostretching.

Embodiment 39. A PTFE layer comprising a layer made by

-   -   providing a layer of PTFE;    -   applying a stretching agent to a surface of the layer; and    -   stretching the layer of PTFE while the layer of PTFE is wet with        the stretching agent.

Embodiment 40. A PTFE layer comprising a layer made by

-   -   providing a layer of PTFE;    -   applying a stretching agent to a surface of the layer;    -   stretching the layer of PTFE while the layer of PTFE is wet with        the stretching agent; and    -   stretching the stretched layer of PTFE a second time.

Embodiment 41. A PTFE layer comprising a layer made by

-   -   providing a layer of PTFE;    -   applying a stretching agent to a surface of the layer;    -   stretching the layer of PTFE while the layer of PTFE is wet with        the stretching agent;    -   stretching the stretched layer of PTFE a second time; and    -   calendering the twice-stretched layer of PTFE so as to densify        and further thin the twice-stretched layer of PTFE.

Embodiment 42. A thin PTFE layer comprising substantially low porosity,low permeability, no discernable node and fibril structure, and having athickness of about 0.00005 inch to about 0.005 inch.

Embodiment 43. A composite PTFE film comprising:

-   -   a first layer comprising a stretched layer of PTFE that has a        closed cell microstructure with a plurality of interconnected        high density regions having no discernable node and fibril        microstructure between the high density regions; and    -   a second layer of expanded PTFE which is secured to the first        layer and which includes a substantial node and fibril        microstructure.

Embodiment 44. A thin, substantially liquid-impermeable PTFE layerproduced by:

-   -   providing a PTFE layer;    -   adding a stretching agent to the PTFE layer; and    -   stretching the PTFE layer in at least one direction to reduce a        thickness of the PTFE layer without substantially creating a        liquid permeability in the stretched PTFE layer.

Embodiment 45. The thin layer of embodiment 41 wherein the stretchedPTFE layer comprises a closed cell microstructure that comprises aplurality of interconnected high density regions with no discernablenode and fibril microstructure.

Embodiment 46. A multi-layered vascular graft comprising:

-   -   a first tubular body having an outer surface and an inner        surface that defines an inner lumen of the vascular graft; and    -   a second tubular body having an outer surface and an inner        surface coupled to the outer surface of the first tubular body,    -   wherein one of the first tubular body and the second tubular        body comprises a fluid-permeable PTFE layer and the other        tubular body comprises a PTFE layer having low fluid        permeability.

Embodiment 47. The multi-layered vascular graft of embodiment 46 whereinthe PTFE layer having low fluid permeability comprises a closed cellmicrostructure that comprises a plurality of interconnected high densityregions wherein the closed cell microstructure has no discernable nodeand fibril microstructure.

Embodiment 48. The multi-layered vascular graft of embodiment 46 whereinthe PTFE layer having low fluid permeability comprises a thin PTFE layerhaving substantially low porosity, no discernable node and fibrilstructure, and a high degree of limpness and suppleness so to allowmechanical manipulation or strain of the PTFE layer without significantrecoil or spring back.

Embodiment 49. An inflatable endovascular graft comprising a bodyportion having an inflatable channel that defines an inflatable space,wherein the inflatable space is at least partially surrounded by a thin,PTFE layer having substantially no fluid permeability.

Embodiment 50. The inflatable endovascular graft of embodiment 49wherein the PTFE layer having substantially no fluid permeabilitycomprises a closed cell microstructure that comprises a plurality ofinterconnected high density regions and wherein the closed cellmicrostructure has no discernable node and fibril microstructure.

Embodiment 51. The inflatable endovascular graft of embodiment 49wherein the PTFE layer having substantially no fluid permeabilitycomprises a thin PTFE layer having substantially low porosity, nodiscernable node and fibril structure, and a high degree of limpness andsuppleness to allow mechanical manipulation or strain of the PTFE layerwithout significant recoil or spring back.

Embodiment 52. A stretched, PTFE layer that comprises a closed cellmicrostructure having high density regions whose grain boundaries aredirectly interconnected to grain boundaries of adjacent high densityregions and having no discernable node and fibril microstructure andhaving substantially no fluid permeability.

Embodiment 53. A composite film comprising a fluid-permeable, expandedPTFE layer secured to a surface of a thin stretched PTFE layer having aclosed cell microstructure having high density regions whose grainboundaries are directly interconnected to grain boundaries of adjacenthigh density regions and having no discernable node and fibrilmicrostructure.

Embodiment 54. A tubular structure comprising a composite filmcomprising a fluid-permeable, expanded PTFE layer secured to a surfaceof a thin, stretched PTFE layer having a closed cell microstructurehaving high density regions whose grain boundaries are directlyinterconnected to grain boundaries of adjacent high density regions andhaving no discernable node and fibril microstructure.

Embodiment 55. An endovascular graft comprising a composite film with afluid-permeable, expanded PTFE layer secured to a surface of a thinstretched PTFE layer having a closed cell microstructure having highdensity regions whose grain boundaries are directly interconnected tograin boundaries of adjacent high density regions and having nodiscernable node and fibril microstructure.

Embodiment 56. A thin PTFE layer, comprising substantially low porosity,low liquid permeability, no discernable node and fibril structure, and ahigh degree of limpness and suppleness so to allow mechanicalmanipulation or strain of the PTFE layer without significant recoil orspring back.

Embodiment 57. A thin layer of PTFE comprising a stretched layer of PTFEthat has a closed cell microstructure with a plurality of interconnectedhigh density regions having no discernable node and fibrilmicrostructure between the high density regions.

Embodiment 58. A method of controlling the porosity, density or both ofa PTFE layer, comprising:

-   -   stretching the PTFE layer at least one time at a preselected        temperature while using a preselected stretching agent content        for the at least one stretch.

What is claimed is:
 1. A composite film comprising: a first layercomprising: a material comprising polytetrafluoroethylene; and a closedcell microstructure with a plurality of interconnected high-densityregions of polytetrafluoroethylene, the closed cell microstructure beingsubstantially free of a node and fibril microstructure between thehigh-density regions; and a second layer of expandedpolytetrafluoroethylene having a substantial node and fibrilmicrostructure secured to the first layer, the first layer and thesecond layer defining an inflatable space therebetween.
 2. The compositefilm of claim 1, wherein the first layer has a thickness from 0.00005inch to 0.005 inch.
 3. The composite of claim 1, wherein the first layerhas substantially no fluid permeability with a Gurley Number of greaterthan 12 hours for 100 cc of air at a pressure of 12.4 cm of water or alimited fluid permeability with a Gurley Number of greater than 1,500seconds for 100 cc of air at a pressure of 12.4 cm of water.
 4. Thecomposite film of claim 1, wherein the material comprises a copolymer ofpolytetrafluoroethylene.
 5. The composite film of claim 4, wherein thecopolymer of polytetrafluoroethylene is a fluorinated copolymer ofpolytetrafluoroethylene.
 6. The composite film of claim 4, wherein thecopolymer of polytetrafluoroethylene isperfluoropropylvinylether-modifiedpolytetrafluoroethylene.
 7. Thecomposite film of claim 4, wherein the copolymer ofpolytetrafluroethylene is about 2% or less of the mass of the material.8. A tubular structure comprising: a composite film having a first layerand a second layer, the first layer comprising: a material comprisingpolytetrafluoroethylene; and a closed cell microstructure with aplurality of interconnected high-density regions ofpolytetrafluoroethylene, the closed cell microstructure beingsubstantially free of a node and fibril microstructure between thehigh-density regions, and the second layer comprising: a materialcomprising expanded polytetrafluoroethylene; and a substantial node andfibril microstructure, wherein the second layer is secured to the firstlayer, and an inflatable space is defined by the space therebetween. 9.The structure of claim 8, wherein the tubular structure is a portion ofa vascular graft.
 10. The structure of claim 8, wherein the first layercomprises a thickness from 0.00005 inch to 0.005 inch.
 11. The structureof claim 8, wherein the first layer has a limited or substantially nofluid permeability.
 12. The structure of claim 11, wherein the firstlayer has a Gurley Number of greater than 12 hours for 100 cc of air ata pressure of 12.4 cm of water.
 13. The structure of claim 11, whereinthe first layer has a Gurley Number of greater than 1,500 seconds for100 cc of air at a pressure of 12.4 cm of water.
 14. The structure ofclaim 8, wherein the material comprises a copolymer ofpolytetrafluoroethylene.
 15. The structure of claim 14, wherein thecopolymer of polytetrafluoroethylene is a fluorinated copolymer ofpolytetrafluoroethylene.
 16. The structure of claim 14, wherein thecopolymer of polytetrafluoroethylene isperfluoropropylvinylether-modified polytetrafluoroethylene.
 17. Thestructure of claim 14, wherein the copolymer of polytetrafluoroethyleneis about 2% or less of the mass of the material.
 18. The structure ofclaim 14, wherein the copolymer of polytetrafluoroethylene isperfluoropropylvinylether-modified polytetrafluoroethylene.
 19. Acomposite film comprising: a first layer comprisingpolytetrafluoroethylene, the first layer configured to act as a barrierlayer to prevent liquid from passing from one side of the first layer tothe opposite side of the first layer; and a second layer comprisingexpanded polytetrafluoroethylene, the second layer configured to allow aliquid to diffuse therethrough, wherein the second layer is secured tothe first layer, and an inflatable space is defined by the spacetherebetween.
 20. The composite film of claim 19, wherein the compositefilm is a portion of a vascular graft, and wherein the vascular graft isconfigured such that a liquid comprising a therapeutic agent is able todiffuse from the inflatable space through the second layer in apredetermined direction.